Vehicles may be equipped with a speedometer for indicating the vehicle speed to the driver, a prefixed speed control device for operating the vehicle at a designated speed, an automatic gear change assembly, an anti-skid control device, other electrical devices which augment and control the vehicles operation and a vehicle speed pulse generator which generates one electrical pulse per rotation of a shaft of a vehicle through a small angle. Electrical pulses may be converted into an analog vehicle speed signal by means of an F/V convertor or may be converted into a digital signal by counting the number of electrical pulses generated in a specified time and calculating the cycle of the electrical pulse, thus obtaining overall vehicle speed information. For the vehicle speedometer, one end of a cable is connected to the output axis of a transfer unit while the other end is connected to a generator or rotary encoder. There are many different types of rotary encoders such as a slit type rotator comprised of a light transparent window or a light reflector which may be detected by a photosensor or one utilizing magnetically sensitive material such as a switch or pick up coil located adjacent a rotating permanent magnet. There is also a possibility to have the output axis of the vehicle transmission connected to a rotary encoder which is separate from the vehicle speedometer cable. In any case, when the vehicle pulse generator malfunctions due to a problem such as a cut wire, a short circuit or the like, the vehicle speed indicator or vehicle related controls will also malfunction. In the past two sets of vehicle pulse generators were provided and the vehicle speed was calculated by the electrical pulses generated by each pulse generator and the comparison was made of the pulses. When the difference exceeded a certain level, the vehicle speed generator which generated the lower vehicle speed pulse was considered abnormal and an alarm was activated.
Assuming one of the vehicle speed pulse generators is connected with the photosensor type rotary encoder and the other vehicle speed pulse generator is connected with a permanent magnet type rotary encoder, it is well known that when the vehicle operates at very low speeds the latter would not generate a substantial pulse while the former would generate a pulse. It has been determined that the pulse was not generated by the permanent magnet type rotary encoder at lower speeds, especially during the start up stage, due to a disturbance imposed upon the rotation of the permanent magnet by the magnetic flux of the permanent magnet especially in combination with a pick up coil wherein the inductive potential of the pick up coil at extremely low speeds of rotation is so low that a pulse is not detected. As a result a false signal of abnormality would arise and the present invention is designed to prevent the occurrence of such false abnormality signals.